Commit 1f6f723c missed a step.  At first I was worried that scrubbing
the conditionals would lead to a runtime failure when compiled without
avahi, but my testing makes it appear that the runtime error will only
occur if the .conf files in /etc request mdns advertisement; and the
old behavior was to silently ignore the request, so this is actually
a better behavior of only failing when the config requests the
impossible.

* src/rpc/virnetserver.c: Drop HAVE_AVAHI conditionals; all
callers already passed NULL if mdns_adv was not configured.